In lithographic printing systems, resolution is an important performance criteria. Resolution generally describes the ability of an optical system to distinguish between closely spaced objects. The resolution parameter of an optical system in lithography systems is important because it is one of the primary limitations in minimizing transistor device sizes. Presently, the quality of imaging systems is quite high so that the imaging characteristics are primarily limited by diffraction effects and therefore are often called "diffraction limited systems."
The manner in which the diffraction phenomena impacts lithography systems is illustrated in prior art FIG. 1a. Prior art FIG. 1a illustrates an illumination intensity distribution produced at a surface or substrate by a spatially coherent light source as it passes an edge such as a pattern on a reticle or mask. Ideally, as illustrated in prior art FIG. 1b, the illumination intensity profile at the substrate, where a pattern is to be transferred would appear as a step function, wherein a region beneath the reticle pattern or feature (e.g., an intended non-illumination region) would not be illuminated (e.g., a transmission=0), while the region not masked by the reticle feature would be fully illuminated (e.g., transmission=1). Diffraction effects, however, as illustrated in prior art FIG. 1a, cause the light radiation to spread into a region intended to be masked while a region near the reticle pattern edge which is to be fully illuminated experiences an attenuated illumination intensity. Furthermore, the intensity distribution in the illuminated region may be a series of alternating light and dark bands, and is dependent on the distance between the reticle and the substrate being exposed as well as the geometry of the slit (e.g., the region or space between neighboring features on the mask or reticle) through with the radiation travels.
Prior art FIGS. 2a-2c illustrate Fraunhofer diffraction which is experienced in most projection printing lithography systems where the distance between the reticle or mask and the substrate is substantial (as opposed to contact or proximity type printing systems). In FIG. 2a, plane waves of radiation encounter a slit having a width "w" and exhibit diffraction, resulting in an illumination intensity profile at a substrate or surface that is a function of the slit width "w" and the radiation wavelength .lambda.. As can be seen in prior art FIGS. 2b and 2c, as the patterns on a mask or reticle get smaller to achieve smaller features on the substrate, the slit width "w" between features on the mask or reticle decreases which causes increased diffraction and decreased resolution at the substrate or surface.
One prior art solution to improving the resolution of a lithographic printing system involves mask design and is illustrated in prior art FIGS. 3a and 3b. Prior art FIG. 3a illustrates a fragmentary cross section of a traditional mask 10 having a transmissive substrate 12 such as glass and opaque features 14 thereon such as chrome. Radiation (e.g., ultraviolet (UV) light) passes through the transparent apertures 16 between the features 14 to expose, for example, a wafer substrate therebeneath (not shown) to thus transfer the mask or reticle pattern to the wafer substrate. At the mask 10, the electric field (E.sub.mask) corresponding to the illumination intensity pattern has the same phase at each of the apertures 16.
Diffraction and the imperfect resolution of the optical system, however, causes the electric field pattern to spread into the intended non-illuminated regions 18 at the wafer substrate (E.sub.substrate). Constructive interference of the electric field patterns at each aperture 16 enhances the electric field pattern underneath the features 14 at the wafer substrate. Since the illumination intensity pattern at the wafer substrate (I.sub.substrate) is proportional to the square of the electric field (E.sub.substrate), the illumination contrast at the wafer substrate between intended illuminated regions 19 and non-illuminated regions 18 is poor.
Prior art FIG. 3b illustrates a fragmentary cross section of a phase shift mask 20 having the substrate 12 and the opaque features 14 thereon. The apertures of the phase shift mask 20, however, differ from the apertures 16 of prior art FIG. 3a. In prior art FIG. 3b, a first aperture 22 is a traditional aperture while a second, neighboring aperture 24 has a portion 26 of the substrate 12 removed. If the depth of the aperture 24 is selected to be one-half of the radiation wavelength (.lambda./2), the phase of the electric field at the mask 20 (E.sub.mask) will be 180.degree. out of phase with the first aperture 22, as illustrated in prior art FIG. 3b.
Although the illumination intensity at the mask surface is identical to the traditional mask 10, destructive interference caused by the diffraction at the features 14 serves to minimize the electric field at the wafer substrate (E.sub.substrate) in the desired non-illumination regions 18. Again, since the illumination intensity at the wafer substrate (I.sub.substrate) is proportional to the square of the electric field (E.sub.substrate),in this case the illumination intensity contrast between the intended illuminated regions 19 and non-illuminated regions 18 at the wafer substrate is substantially improved.
Although the phase shift mask 20 of prior art FIG. 3b exhibits substantially better performance than the traditional mask 10 of prior art FIG. 3a, diffraction effects still cause the non-illuminated regions 18 to be partially exposed. As features continue to decrease, however, the percentage of the intended non-illuminated region 18 which receives illumination continues to increase until eventually the entire region is illuminated and feature resolution is lost. Thus, although the phase shift mask 20 provides an improvement over the traditional mask 10, it too encounters difficulties in maintaining sufficient resolution as feature sizes continue to shrink.
In addition to difficulties in resolution as feature sizes continue to shrink, the phase shift mask 20 of FIG. 3b also suffers from limited focus process latitude. As discussed above, the phase shift mask 20 provides optimal destructive interference at a particular focal plane below the mask 20. Therefore it is desirable to locate the wafer substrate so that it coincides with that focal plane. If the surface of the wafer substrate deviates from the focal plane, the optimal destructive interference is not achieved and the system resolution is substantially degraded. The focus process latitude parameter is therefore an indication of how much variation from the optimal focal plane is permitted before sufficient resolution is lost.
Several factors may cause a wafer to be located off of the optimal focal plane, as illustrated in prior art FIG. 4. A mask 30 is preferably located a distance "d" away from a wafer 32 which is covered with a photosensitive film 34 such as a photoresist. The distance "d" represents the optimal focal plane at which optimal destructive interference exists in the intended non-illuminated regions 18 at the photoresist 34. The distance "d", although optimal, is typically not achieved, but rather varies by .+-..DELTA.d so that the actual distance between the mask 30 and the film 34 is d.+-..DELTA.d. The variation .+-..DELTA.d may be due to variations in the wafer thickness .+-..DELTA.t.sub.1 from wafer to wafer as well as variations within a wafer due to non-planarity, etc. In addition, the variation .+-..DELTA.d may be due to variations in the resist thickness .+-..DELTA.t.sub.2 which can vary .+-.300 nm or more. Thus in order to maintain good system resolution, it is desirable to have a mask design which provides increased focus process latitude throughout the entire range of distances d.+-..DELTA.d that may be experienced during typical processing.